jfs: jfs_dmap: Validate db_l2nbperpage while mounting

In jfs_dmap.c at line 381, BLKTODMAP is used to get a logical block number inside dbFree(). db_l2nbperpage, which is the log2 number of blocks per page, is passed as an argument to BLKTODMAP which uses it for shifting.

Syzbot reported a shift out-of-bounds crash because db_l2nbperpage is too big. This happens because the large value is set without any validation in dbMount() at line 181.

Thus, make sure that db_l2nbperpage is correct while mounting.

Max number of blocks per page = Page size / Min block size => log2(Max num_block per page) = log2(Page size / Min block size)
= log2(Page size) - log2(Min block size)

=> Max db_l2nbperpage = L2PSIZE - L2MINBLOCKSIZE

Reported-and-tested-by [email protected]

Closes https://syzkaller.appspot.com/bug?id=2a70a453331db32ed491f5cbb07e81bf2d225715

Cc [email protected]

Suggested-by Dave Kleikamp <[email protected]>

Signed-off-by Siddh Raman Pant <[email protected]>

---
Changes in v3:
- Fix typo in commit message (number of pages -> number of blocks per page).

Changes in v2:
- Fix upper bound as pointed out in v1 by Shaggy.
- Add an explanation for the same in commit message for completeness.

  fs/jfs/jfs_dmap.c   | 6 ++++++
  fs/jfs/jfs_filsys.h | 2 ++
  2 files changed, 8 insertions(+)

fs/jfs/jfs_dmap.c +6 -0

index a3eb1e826947..da6a2bc6bf02 100644
@@ -178,7 +178,13 @@ int dbMount(struct inode *ipbmap)
  	dbmp_le = (struct dbmap_disk *) mp->data;
  	bmp->db_mapsize = le64_to_cpu(dbmp_le->dn_mapsize);
  	bmp->db_nfree = le64_to_cpu(dbmp_le->dn_nfree);
+
  	bmp->db_l2nbperpage = le32_to_cpu(dbmp_le->dn_l2nbperpage);
+	if (bmp->db_l2nbperpage > L2PSIZE - L2MINBLOCKSIZE) {
+		err = -EINVAL;
+		goto err_release_metapage;
+	}
+
  	bmp->db_numag = le32_to_cpu(dbmp_le->dn_numag);
  	if (!bmp->db_numag) {
  		err = -EINVAL;

fs/jfs/jfs_filsys.h +2 -0

index b5d702df7111..33ef13a0b110 100644
@@ -122,7 +122,9 @@
  #define NUM_INODE_PER_IAG	INOSPERIAG
 
  #define MINBLOCKSIZE		512
+#define L2MINBLOCKSIZE		9
  #define MAXBLOCKSIZE		4096
+#define L2MAXBLOCKSIZE		12
  #define	MAXFILESIZE		((s64)1 << 52)
 
  #define JFS_LINK_MAX		0xffffffff